Method and apparatus for automatically controlling temperature in a furnace system

ABSTRACT

A method and system for controlling the temperature in a furnace such as a continuous feed controlled atmosphere furnace includes an upstream thermocouple in a first area of the furnace, a second thermocouple in a second area of the furnace, and a heater control system. A sensed temperature from the upstream thermocouple is used to generate a control signal for a heater associated with the first area of the furnace. The sensed temperature from the first area of the furnace is further used to modify a set point that is compared to a sensed temperature from the second thermocouple. A control signal for a heater associated with the second area of the furnace is generated based upon the comparison of the modified set point to the sensed temperature from the second thermocouple.

FIELD OF THE INVENTION

The present invention relates generally to the field of furnace systems and, in particular, to an automatic temperature control system for a furnace.

BACKGROUND OF THE INVENTION

Brazing is a commonly used technique for joining metal parts with close fitting joints. Typically, a clad material that has a flux cleaning solution applied is melted within a furnace or oven allowing the clad to flow into the gap between adjacent parts. As the clad flows into the gap, the flux cleaning solution cleans the adjacent parts allowing the clad material to be strongly bonded to the adjacent parts. Many commercial brazing operations are carried out on a continuous conveyor belt that passes through heated sections, or furnaces, of the brazing system. The conveyer belt or carrier belt is designed with a sufficient length to raise the part to be brazed to a brazing temperature, and to maintain the part at the desired temperature for a length of time sufficient to braze the part.

The furnaces are usually fitted with a muffle disposed within a refractory structure. A muffle is a tube that extends along the length of the furnace. The muffle is used to maintain a desired atmosphere surrounding the part to be brazed. The atmosphere is selected to protect the surface from contaminants. For example, when brazing aluminum components the presence of oxygen may result in oxidation or coloration at the surfaces being brazed, which results in an unacceptable braze. Accordingly, the controlled atmosphere when brazing aluminum components is typically maintained by continuously pumping nitrogen into the muffle. Thus, the area within the muffle is maintained at with a positive pressure of the selected atmosphere with respect to the area outside of the muffle.

In a typical brazing process, the part to be brazed is fluxed and the components to be brazed are placed in the desired position in contact with each other. In order to ensure an optimum braze, it is first necessary to eliminate any moisture from the metal parts of the flux. Accordingly, most brazing systems include a dry-off or dehydration oven between the fluxer and the controlled atmosphere furnaces. The purpose of the dry-off oven is to raise the core part temperature sufficiently high to ensure that all moisture has been evaporated from the part. Typically, a dry-off oven will raise the part temperature to about 180° C. (350° F.) in an air atmosphere.

After the components have been dried, the carrier belt carries the components into the muffle area. The muffle is heated by the use of banks of natural gas and/or electric heating elements outside the muffle. The heating elements are located outside of the muffle area to reduce leakage of the controlled atmosphere. The heating elements are in turn located within an outer casing or containment that provides insulation. The insulated casing allows the area within the casing, including the muffle area, to be heated significantly above ambient temperature in a reasonably efficient manner. In a typical aluminum brazing operation, the temperature within the muffle is desired to be about 590° C. (1100° F.).

Thermocouples are typically used to provide indications of the temperature within the muffle and/or temperatures near the heating banks. The indicated temperatures are used to control the external heating elements. If used, thermocouples measuring the actual heating element temperatures are located within thermocouple wells that penetrate the insulated casing and terminate outside of the muffle next to the heating elements. Thermocouples that are provided to obtain an indication of the temperature within the muffle area may be located within wells that penetrate the muffle wall.

As the components enter into and move through the muffle area, the components, along with carrier belt and any required fixturing used to maintain the components in the desired position are heated to the brazing temperature. The foregoing items thus act as heat sinks, absorbing the heat energy within the muffle and causing the temperature within the muffle to be lowered. This cooling effect is not felt evenly throughout the muffle, as the work pieces will typically be at a lower temperature as they enter the first zone. Thus, there is more cooling toward the front or upstream end of the furnace. Accordingly, it is known to locate different banks of heaters in different locations about the muffle and to operate the heater banks independently of each other. Zone heating systems are based upon this type of an approach.

In zone heating, the heater banks are divided into controlled banks along the length of the carrier belt. Each zone is then independently controlled. Thus, more heat can be injected into the upstream zones while maintaining the downstream zones at the desired temperature while injecting less heat into the downstream portion of the furnace.

Of course, the work pieces are typically not symmetrical. Thus, the temperature in the areas nearest to the items or parts of items presenting the largest heat sink are affected more intensely than the temperature in areas that are closer to smaller heat sinks. Therefore, because the heater banks within a zone typically include upper heater banks and lower heater banks it is known to independently control the upper heater banks and the lower heater banks. The independent control of the upper and lower banks, particularly after the work pieces have been heated to some extent, provides additional efficiencies as it is possible to focus any increased heating within the muffle to those zones and areas of zones that are actually cooling the most.

While this method of temperature control is effective, there are problems associated with the method. For example, the brazing operation is very sensitive to fluctuations in heat. Thus, to maintain a minimum temperature within a zone, the zone must be kept above the needed temperature and controlled at that higher set point so that the temperature does not fall too far below the set point. This set point offset is established based upon the ability of the system to respond to a cooling event. Accordingly, the set point offset is a function of the size of components passing through the muffle, the initial temperature of the components, the speed of the carrier belt, the heating capacity of the heating bank, and the delay associated with the use of thermocouples and external heaters. This delay is referred to as temperature lag.

Temperature lag is the time that it takes for a change in temperature to be sensed and corrected. With reference to a cooling event, because the thermocouples are located within wells, the well wall must cool before the thermocouple senses the drop in temperature. Accordingly, as temperature in the muffle cools, a thermocouple reading does not indicate the actual temperature. Rather, the thermocouple will generally indicate that the temperature in the muffle area is warmer than the actual temperature in the muffle area until a steady state value is achieved. Thus, by time a temperature deviation is sensed that triggers activation or increased activation of a heater bank or element, the temperature within the muffle area may have fallen to yet a lower temperature. The temperature lag problem is compounded by the fact that the heat generated by the external heaters must propagate through the muffle before heating the component with radiant heat. Therefore, temperature within the muffle may continue to fall until the component passes out of the area proximate to the heater bank or until the heater bank generates sufficient heat through the muffle to offset the cooling effect of the components.

Therefore, when designing a heater control system, the temperature offset is selected to account for the above factors and maintain a higher temperature than is needed for the actual brazing operation. Of course, the higher temperature introduces additional inefficiency into the system. Moreover, as zone size increases, inefficiencies may further increase since the entire zone is controlled based upon a decreased temperature in one area of the muffle zone.

Accordingly, although zone heat control provides an improvement in efficiency, the system still suffers from inefficiencies related to the increased set point offset within each zone. The established set point may of course be reduced in a number of ways. However, each of these methods has undesired consequences. For example, by lowering the speed of the carrier belt through the muffle, any localized cooling will occur at a slower rate. Thus, the set point may be established at a lower temperature while ensuring that the components do not fall below the minimum brazing temperature. However, this results in an increased brazing time per component and reduced furnace throughput. Alternatively, the amount of heat that can be injected into the zone by the external heating banks can be increased. However, the additional heaters introduce increased capital expenditures.

What is needed therefore is a system and method that allows for a reduced set point offset without increasing brazing time per component. It would be beneficial if the method and system did not require additional heating banks or heating banks of larger heat generating capacity. It would be further beneficial if the method and system incorporated the advantages of zone heating control. It would also be beneficial if the method and system reduced the magnitude of temperature excursions above and/or below the temperature set point for the furnace and/or zone.

SUMMARY OF THE INVENTION

In order to address these needs, the present invention contemplates a method and system for controlling a muffle temperature that predicts heat load within heating zones. A heating zone for the muffle is provided with a first and a second thermocouple, the second thermocouple located downstream of the first thermocouple. The temperature in the muffle is sensed by the upstream thermocouple. This indication of temperature is used to control an upstream bank of heaters or heating elements. The temperature indication from the first thermocouple is also used to modify the temperature set point used to control a bank of heaters or a heating element downstream of the first thermocouple.

In one embodiment, when a downward temperature excursion is sensed by the first thermocouple, the set point used to control the downstream bank of heaters is increased. Thus, more heat energy is injected into the muffle area proximate the downstream bank of heaters. Accordingly, by the time the components causing the cooling within the muffle reach the area proximate the second heating bank, the second heating bank is already energized and additional heat energy is being injected into the muffle. Thus, any downward excursion in the temperature within the muffle below the original set point used to control the bank of heaters downstream of the first thermocouple is reduced.

In another embodiment, a first set point is defined for use in controlling the temperature in a first area of a furnace and a second set point is defined for controlling the temperature in a second area of the furnace. A high deviation set point and a low deviation set point are also defined. A controller is programmed to compare sensed temperature in a first area of a furnace to the high deviation set point and a low deviation set point and to increase the second set point if a low deviation condition is sensed in the first area and to decrease the set point if a high deviation condition is sensed in the first area.

One benefit of the present invention is that temperature excursions above and/or below the temperature set point for the furnace and or zone are minimized by increasing or decreasing the amount of energy being introduced into a portion of the furnace in anticipation of an increased or decreased need for energy in that portion of the furnace. Another benefit of the present invention is that the effects of temperature lag in a muffle furnace are minimized. It is a further benefit that the above advantages may be realized without increasing the number or sizes of heating elements.

Other objects and benefits of the present invention will become apparent upon consideration of the following written description, taken together with the accompanying figures.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a general perspective view of one type of continuous brazing system incorporating the heater control system of one embodiment of the present invention.

FIG. 2 shows a cross sectional view of the brazing furnace of the continuous brazing system shown in FIG. 1.

FIG. 3 shows a general perspective view of a zone of the brazing furnace of the continuous brazing system shown in FIG. 1 with the furnace containment removed for purposes of description.

FIG. 4 shows a simplified diagram of the temperature control circuit used with the continuous brazing system shown in FIG. 1 in accordance with features of the present invention.

FIG. 5 shows a control panel that can be used to monitor and adjust the values in the continuous brazing system of FIG. 1.

FIG. 6 shows the control panel of FIG. 5 with an alternative control field.

FIG. 7 shows a representation of a memory that may be used to store values used by the temperature control circuit of FIG. 4 to control temperature in the continuous brazing system shown in FIG. 1.

FIG. 8 shows a flow diagram of a method that includes features of the invention and that may be programmed into a controller in the temperature control circuit of FIG. 4 to control temperature in the furnace of FIG. 1.

FIG. 9 shows the memory of FIG. 7 with values stored in the memory slots of the memory.

FIG. 10 shows a representation of an external memory that may be used to store values used by the temperature control circuit of FIG. 4 to control temperature in the continuous brazing system shown in FIG. 1.

FIG. 11 shows a flow diagram of an alternative method that includes features of the invention and that may be programmed into a controller in the temperature control circuit of FIG. 4 to control temperature in the furnace of FIG. 1.

FIG. 12 shows the external memory of FIG. 10 with values stored in some of the memory slots of the external memory.

DETAILED DESCRIPTION

For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and described in the following written specification. It is understood that no limitation to the scope of the invention is thereby intended. It is further understood that the present invention includes any alterations and modifications to the illustrated embodiments and includes further applications of the principles of the invention as would normally occur to one of ordinary skill in the art to which this invention pertains.

A continuous brazing system 10, shown in FIG. 1, includes a braze section or brazing furnace 12 at the discharge end of a continuous serial process. The brazing furnace 12 includes a muffle 14 disposed within a refractory furnace containment 16. Stock material and flux are fed into the muffle 14 through an inlet 18 and exit the brazing system 10 through the outlet 20 on a carrier belt 52 shown in FIG. 2. In addition, the system 10 can include a pre-heat section 22 that also includes a muffle 24 within a furnace containment 26. Stock material and flux enter the pre-heat section 22 through the inlet 28 and exit through the outlet 30 to be conveyed to the brazing furnace 12.

As is typical with most continuous brazing systems, the muffles 14 and 24 provide a controlled atmosphere and include means for maintaining that controlled atmosphere within the interior of the muffle. In an aluminum brazing system, the atmosphere is primarily composed of nitrogen. In order to maintain this controlled atmosphere, the system 10 may be provided with a vestibule (not shown) between the outlet 30 of the pre-heat section 22 and the inlet 18 of the brazing furnace 12. Likewise, a vestibule (not shown) can be provided at the inlet 28 of the pre-heat section 22. The vestibules can be of conventional construction.

With the downstream components of the system 10 described, attention can turn to the dry-off oven 32 at the upstream end of the process. The dry-off oven 32 receives stock material and flux after it has left the fluxer (not shown). The dry-off oven 32 includes an inlet 34 and an outlet 36 that provide a path for the material through the dry-off oven 32. It is understood that the dry-off oven 32, as well as the downstream pre-heat section 22 and brazing furnace 12 can be integrated with a continuous conveyor system extending through the respective inlets and outlets.

The dry-off oven 32 can include an exhaust unit 38 that is operable to exhaust spent gas from the chamber 40 of the oven. The exhaust unit 38 can be of a variety of configurations to exhaust the gases from the oven to the atmosphere. The exhaust unit 38 may comprise one or more rotary fans connected to discharge hoods or shrouds at the ends of the oven 32, or more particularly, shrouds situated around the perimeter of the inlet 34 and the outlet 36. The fans can feed the exhaust gas to one or more exhaust stacks outside the building housing the system 10. Typically, a single exhaust blower is ducted to both the discharge and charge hoods and dampers are used to control flow from each hood.

The brazing furnace 12 in this embodiment includes upper and lower heating elements, each element having a thermocouple associated with the element. With reference to FIG. 2, thermocouple 42 is associated with the upper upstream heating element 44 and the thermocouple 46 is associated with the lower upstream heating element 48. The wells 43 and 45 of the thermocouples 42 and 46, respectively, project into the muffle area 50. The thermocouples 42 and 46 are located at about one third of the distance from the inlet 18 to the outlet 20 of the brazing furnace 12. Thus, the thermocouples 42 and 46 are placed in a position to provide an indication of the temperature of an area within the muffle 14 proximate to the upper upstream heating element 44 and the lower upstream heating element 48, respectively.

The brazing system 10 in this embodiment includes a plurality of heating zones, each zone having a plurality of upper and lower heating elements. As shown in FIG. 3, heating zone 54 includes, in addition to the thermocouples 42 and 46 and the upper upstream heating element 44 and lower upstream heating element 48, the thermocouples 56 and 58, the upper downstream heating element 60 and the lower downstream heating element 62. The thermocouples 56 and 58 and the upper downstream heating element 60 and the lower downstream heating element 62 are located at about two thirds of the distance from the inlet 18 to the outlet (not shown) of the heating zone 54. An upper over-temperature thermocouple 64 and lower over-temperature thermocouple 66 are also shown in FIG. 3. The over-temperature thermocouples 64 and 66 may be alternatively located within the zone 54, and more or fewer over-temperature thermocouples may be provided. For example, in one alternative embodiment a single over-temperature thermocouple is located above the muffle and above the upstream thermocouple. The upper upstream portion of a zone will generally be subjected to the highest temperatures. The larger excursions in temperature result from the fact that the upstream portion of the zone is the first portion that sees a reduction is the heat load being introduced into the zone. Thus, a single over-temperature thermocouple in the upper upstream portion of the zone may provide sufficient over-temperature protection.

FIG. 4 shows a simplified diagrammatic representation of the heater control system used with the brazing system 10. The heater control system circuit 68 is used to control the upper upstream heating element 44, the lower upstream heating element 48, the upper downstream heating element 60 and the lower downstream heating element 62. The control circuit 68 includes a controller 70 which in this embodiment is an Allen-Bradley brand PLC 5-30/40 controller commercially available from Rockwell Automation of Milwaukee, Wis. The controller 70 is programmed to provide control signals to the heater banks in response to sensed temperatures from the brazing furnace 12 using a proportional-integral-derivative (PID) function. The control operation of the controller 70 is described more fully below. The controller 70 includes a plurality of input/output modules 72, 74, 76 and 78. The input/output modules 72, 74, 76 and 78 are model 1771-TCM temperature control modules also available from Rockwell Automation.

The control circuit 68 also includes the silicon controlled rectifiers (SCRs) 80, 82, 84 and 86. The SCRs 80, 82, 84 and 86 in this embodiment are model TC2000 SCRs commercially available from Eurotherm, Inc. of Leesburg, Va. The SCRs 80, 82, 84 and 86 provide power to the upper upstream heating element 44, the upper downstream heating element 60, the lower upstream heating element 48 and the lower downstream heating element 62, respectively. Power to the SCRs 80, 82, 84 and 86 may be interrupted by the contacts 88, 90, 92 and 94, respectively. The contacts 88 and 90 are controlled by the control instrument 96 which receives input from the upper over-temperature thermocouple 64 while the contacts 92 and 94 are controlled by the control instrument 98 which receives input from the lower over-temperature thermocouple 66. The controller 70 is also connected to the contacts 88, 90, 92 and 94 such that an enable or reset signal may be issued from the controller 70 to shut the contacts 88, 90, 92 and 94.

A user interface that may be used in accordance with one embodiment of the invention is shown in FIG. 5. The control panel 100 includes a furnace status field 102, navigation buttons 104, 106, 108, 110 and 112 and a zone control area 114. The furnace status field 102 shows the temperature set point that has been established by the user to control the temperature of each of the zones 2, 3, 4, 5, 6, 7, 8 and 9. In this embodiment, the user may establish a separate set point for the upper and the lower portion of zones 4-9. Accordingly, an upper and a lower set point value are shown for each of zones 4-9. The furnace status field 102 also shows the presently sensed temperature within each of the zones as indicated by the rows labeled “Process Variable”. A separate reading of the temperature within the zones 4-9 is provided in the “Muffle” row. This temperature may be obtained from a thermocouple located at a position in the zone that is most likely to display the highest temperature.

The navigation buttons 104, 106, 108 and 110 are used to change the subject matter being displayed in the zone control area 114. Thus, a user can select any of the zones 2-9 for display in the zone control area 114. The navigation button 112 is used to navigate to a pan control view of the brazing system 10 from which navigation to control panels for the various portions of the brazing system 10 is possible.

The zone control area 114 shown in FIG. 5 includes a separate upper control window 116 and lower control window 118. Each of the upper control window 116 and lower control window 118 include a display area 120 and 122, respectively, that shows the present set point, temperature, and power output of the respective heater bank as a percentage. The upper control window 116 further includes the change value buttons 124, 126, and 128 while the lower control window 118 includes the change value buttons 130, 132 and 134.

The change value buttons are used to provide various inputs that are used in controlling the heater banks. For example, change value button 124 is used to provide a manual override of the normal PID function of the controller 70. By operation of the change value button 124, a percentage value is input into the control system for use by the controller 70 and the heater bank output is then controlled to the set percentage value. The change value button 124 is typically used in calibrating the output of the controller 70 to the SCRs 80, 82, 84 and 86.

Activation of the change value button 128 causes a keypad to be displayed which may be used to modify the temperature set point used to control the temperature in the displayed zone. Activation of the heat enable button 121 causes the controller 70 to output a signal to the contacts 88, 90, 92 and 94. The signal from the controller 70 is used to shut the contacts 88, 90, 92 and 94 to allow power to be sent to the respective heater elements. For example, the heat enable button 121 is used to reset the contacts 88, 90, 92 and 94 after an over-temperature condition, discussed more fully below, has cleared.

Another control panel that is provided for control of the temperature within the brazing furnace 12 is the automatic control input window 136 shown in FIG. 6. The automatic control input window 136 includes the deviation high set point button 138 and the deviation low set point button 140. Activation of the deviation high set point button 138 or the deviation low set point button 140 causes a keypad to be displayed which may be used to key in a set point temperature value. The automatic control input window 136 also includes the deviation high subtract amount button 142 and the deviation low add amount button 144 which are used to modify the set point value defined in the display area 120. The deviation high subtract amount button 142 is used to define the amount by which the set point discussed in reference to the change value button 128 is reduced when the temperature in the zone exceeds the deviation high set point defined using the deviation high set point button 138. The deviation low add amount button 144 is used to define the amount by which the set point discussed in reference to the change value button 128 is increased when the temperature in the zone drops below the deviation low set point defined using the deviation low set point button 140. The operation of the control circuit 68 is set forth more fully below.

In one embodiment of the invention, the various parameters input by the operator are stored in a memory for use by the controller 70. The memory may be located within the controller or in a memory external to the controller. As shown in FIG. 7, memory 146 includes memory slots M₁-M₅, identified by reference numbers 148, 150, 152, 154 and 156, respectively. The memory slot 148 is used to store the set point used by a first PID function in the controller 70 in controlling an upstream heater bank and a second PID function in the controller 70 in controlling a downstream heater bank. The memory slot 150 is used to store a high temperature deviation or offset used to determine when a high deviation temperature condition is present. The memory slot 152 is used to store the value by which the set point entered into the memory slot 148 is to be reduced when controlling a downstream heater in the event of a high temperature deviation condition. The memory slot 154 is used to store a low temperature deviation value or offset used to determine when a low deviation temperature condition is present. The memory slot 156 is used to store the value by which the set point entered into the memory slot 148 is to be increased when controlling a downstream heater in the event of a low temperature deviation condition.

Operation of the control circuit 68 is described with reference to FIG. 8 and the foregoing figures. FIG. 8 shows a control cycle 158 that may be programmed into the controller 70 and may be used to control the power generated by an upstream and downstream heater bank in accordance with principles of the present invention. At step 160, the control cycle 158 is initialized and values set by an operator using the control panel 100 are stored in the memory 146. The control cycle 158 then begins at step 162 in response to a wake-up call to the controller 70. At step 164 the value of the signal generated by input/output module 72 is read. The signal generated by input/output module 72 is indicative of the temperature in the upper upstream portion of the muffle 14 as sensed by the upper upstream thermocouple 42. Of course, while the operation of the control circuit 68 is described as obtaining this and other values at various steps during the process set forth in FIG. 8, those of skill in the relevant art will appreciate that the controller 70 may alternatively be programmed to obtain all the requisite process variables to be used during the control cycle 158 at an early step and freeze the values for the duration of one repetition of the control cycle 158 or until replaced by new values.

Continuing with the operational description, at step 166 the value stored in memory slot 148 is read. The value stored in memory slot 148 is the set point temperature established by the operator. The controller 70 performs a first preprogrammed PID function at step 168 using the set point temperature from memory slot 148 and the signal (process variable) indicative of the temperature in the upper upstream portion of the muffle 14 from the input/output module 72 to generate a first control signal C₁.

The first preprogrammed PID function operates in a typical manner to bring the sensed temperature to the set point temperature, and to then maintain the sensed temperature at the set point temperature as is understood by those of ordinary skill in the relevant art. The first control signal C₁ corresponds to a percentage output power that should be supplied to the upper upstream heating element 44 in order to reach or maintain the set point temperature stored in the memory slot 148. The first control signal C₁accomplishes this by controlling the duty cycle of the SCR 80 when the first control signal C₁ is output to the SCR 80 at step 170. The duty cycle corresponds to a percentage output power that is subsequently supplied to the muffle 14 from the upper upstream heating element 44.

The controller 70 at step 172 then determines if the temperature sensed by the upper upstream thermocouple 42 is greater than the set point temperature stored in the memory slot 148 for the upper upstream area of the muffle 14 plus the high temperature deviation value stored in the memory slot 150 (the deviation high set point). If the sensed temperature is greater than the deviation high set point, then at step 174 the high temperature deviation set point reduction value stored in the memory slot 152 is subtracted from the set point temperature stored in the memory slot 148 with the remainder being the current set point. If the sensed temperature is not greater than the deviation high set point, then at step 176 the sensed temperature is compared to the remainder of the set point temperature stored in the memory slot 148 for the upper upstream area of the muffle 14 minus the low temperature deviation value stored in the memory slot 154 (the deviation low set point). If the sensed temperature is less than the deviation low set point, then at step 178 the low temperature deviation set point increase value stored in the memory slot 156 is added to the set point temperature stored in the memory slot 148 with the sum being the current set point. If neither of the conditions of step 172 or 176 is met, then the set point temperature stored in the memory slot 148 is the current set point.

At step 180, the value of the signal generated by input/output module 74 is read. The signal generated by input/output module 74 is indicative of the temperature in the upper downstream portion of the muffle 14 as sensed by the upper downstream thermocouple 56. The controller 70 then performs the second preprogrammed PID function at step 182 using the current set point temperature and the signal (process variable) indicative of the temperature in the upper downstream portion of the muffle 14 to generate a second control signal C₂. At step 184, the second control signal C₂ is output to the SCR 82. The second control signal C₂ and the SCR 82 operate in a manner similar to that described above with respect to the first control signal C₁ and the SCR 80 so as to control the power output of the upper downstream heating element 60.

At step 186 the control cycle 158 ends until the next wake-up call. Upon receiving a wakeup call, the control cycle 158 begins again at step 162.

The operation of the controller 70 in controlling the lower heating elements is similar to the process described above with respect to the upper heating elements. Moreover, although discussed above in the context of a single zone, the controller 70 may be programmed and connected to other equipment so as to similarly control multiple zones of the brazing furnace 12. Generally, by controlling one or two zones in the upstream area of a furnace, incoming cold work pieces can be quickly brought to a stable brazing temperature. Thus, the potential for low temperature excursions is relatively low in the latter zones. However, the chance for high temperature excursions in the latter zones is not reduced by the same mechanism. Moreover, if desired, both an upper bank and a lower bank of heaters may be controlled using a single thermocouple. All of the above variations are within the scope of the present invention.

The simplified operation of the control circuit 68 is described below as a response to a hypothetical high sensed temperature in the upper upstream portion of the brazing furnace 12 which occurs as a result of the following hypothetical scenario. The brazing furnace 12 has been operating and the memory slots 148-156 have been previously populated with the values shown in FIG. 9. Both the upstream and downstream portions of the muffle 14 are initially at a steady state temperature of 1100 degrees F. with a continuous flow of work pieces through the zone 54. A high temperature excursion is then created by discontinuing placement of work pieces on the conveyer belt 52. Thus, the “cooling” effect of the work pieces is no longer introduced into the brazing furnace 12. Accordingly, as the empty portion of the conveyor belt 52 passes into the upstream portion of the muffle area 14, the temperature in the upstream portion of the muffle increases to 1116 degrees F. after the completion of a control cycle 158.

When the next wake-up call is issued, the controller 70 begins the control cycle 158 at step 162. At step 164 the temperature in the upper upstream portion of the muffle 14 is read as 1116 degrees F. At step 166 the set point temperature established by the operator in memory slot 148 is read as 1100 degrees F. The controller 70 at step 168 performs a first preprogrammed PID function using the set point temperature of 1100 degrees F. from memory slot 148 and the signal (process variable) indicative of the sensed 1116 degrees F. temperature in the upper upstream portion of the muffle 14 to generate a first control signal C₁. Because the sensed temperature is greater than the set point temperature, the first control signal C₁ is generated and output at step 170 so as to shorten the duty cycle of the SCR 80, resulting in a lower power output from the upper upstream heating element 44. This will lead to a lowering of the temperature in the upper upstream portion of the muffle 14 as a result of normal heat loss from the brazing furnace 12.

The controller 70 at step 172 then determines that the 1116 degrees F. temperature sensed by the upper upstream thermocouple 42 is greater than the 1115 degrees F. deviation high set point defined by the 1100 degrees F. set point temperature stored in the memory slot 148 for the upper upstream portion of the muffle 14 plus the 5 degrees F. high deviation value stored in the memory slot 150. Therefore, at step 174 the high temperature deviation set point reduction value of “10” stored in the memory slot 152 is subtracted from the set point temperature stored in the memory slot 148 and the remainder of 1090 degrees F. is selected as the current set point.

At step 180 the value of the signal generated by input/output module 74 is read. The signal generated by input/output module 74 is indicative of an 1100 degrees F. temperature in the upper downstream portion of the muffle 14 as sensed by the upper downstream thermocouple 56. The sensed temperature is still 1100 degrees F. because the downstream portion of the muffle 14 has not yet been affected by the reduction in heat load. Of course, in some situations the sensed temperature may be higher, or an increase in temperature may not yet have been sensed due to temperature lag. The actual situation will vary as a result of design and operational considerations.

The controller 70 then performs the second preprogrammed PID function at step 182 using the 1090 degrees F. current set point temperature and the 1100 degrees F. signal (process variable) indicative of the temperature in the upper downstream portion of the muffle 14 to generate a second control signal C₂. Because the sensed temperature is greater than the current set point temperature, the second control signal C₂ causes the SCR 82 to use a shorter duty cycle, resulting in less power output by the upper downstream heating element 60. At step 186 the control cycle 158 ends until the next wake-up call.

The above process is repeated with the control signals being modified in accordance with sensed changes in temperature until the condition set in step 172 is no longer met or until an over-temperature condition is detected by the control instrument 96 resulting in power to the heater banks being interrupted. Thus, under normal conditions, when the sensed temperature at step 164 reaches 1105 degrees F., then at step 172 the sensed temperature will not be greater than the deviation high set point of 1115 degrees F. Therefore, the controller 70 will proceed to step 176. At step 176, the sensed temperature is compared to the deviation low set point. Because the sensed temperature of 1105 degrees F. is greater than the deviation low set point of 1096 degrees F., the current set point temperature for the downstream portion of the zone is selected as the normal set point temperature stored in the memory slot 148. Thus, at step 182, the second PID function will be performed by the controller using the present temperature as sensed by the upper downstream thermocouple 56 and the 1100 degree F. current set point.

Those of skill in the art will appreciate that in accordance with the forgoing example, the upper downstream portion of the muffle 14 may still experience a temperature excursion above 1100 degrees F. However, the excursion in the upper downstream portion of the muffle 14 will be less than the excursion in the upper upstream portion of the muffle 14. The lessened excursion results since the energy injected into the muffle by the upper downstream heating element 60 was reduced when the event causing the high temperature excursion was affecting the temperature in the upper upstream portion of the muffle 14. Thus, the output of the downstream heating elements may be reduced before the temperature in the downstream portion of the muffle 14 is first affected.

Continuing with the above simplified example, after once again achieving a steady state temperature of 1100 degrees F. in both the upstream and downstream portion of the zone 54, a cooling excursion may be initiated by subsequent loading of the conveyor belt. Accordingly, as the work pieces begin to enter the upstream portion of the muffle 14, the temperature in the upstream portion of the muffle 14 begins to decrease. For purposes of this hypothetical, the temperature decreases to 1095 degrees F. after a control cycle 158 has ended.

Subsequently, a wake-up call is issued and the controller 70 begins the control cycle 158 at step 162. At step 164 the temperature in the upper upstream portion of the muffle 14 is read as 1095 degrees F. At step 166 the set point temperature established by the operator in the memory slot 148 is read as 1100 degrees F. Therefore, at step 168 the controller 70 performs a first preprogrammed PID function using the set point temperature of 1100 degrees F. from the memory slot 148 and the signal (process variable) indicative of the 1095 degrees F. temperature in the upper upstream portion of the muffle 14 to generate a first control signal C₁. Because the sensed temperature is less than the set point temperature, the first control signal C₁ is generated so as to increase the duty cycle of the SCR 80, resulting in a higher power output from the upper upstream heating element 44. This will lead to an increase of the temperature in the upper upstream portion of the muffle 14 or a reduced rate of cooling, depending on the capacity of the heaters and the size of the work pieces.

At step 172 the controller 70 then determines that the 1095 degrees F. temperature sensed by the upper upstream thermocouple 42 is not greater than the 1115 degrees F. deviation high set point. Accordingly, the controller 70 proceeds to step 176 and determines that the 1095 degrees F. temperature sensed by the upper upstream thermocouple 42 is less than the 1096 degrees F. deviation low set point. The controller 70 then proceeds to step 178 and adds the 1100 degrees F. set point temperature stored in the memory slot 148 for the upper upstream area of the muffle 14 and the 10 degrees F. low deviation value stored in the memory slot 156 to define the current set point as 1110 degrees F. At step 182 the value of the signal generated by input/output module 74 is read. The signal generated by input/output module 74 is indicative of an 1100 degrees F. temperature in the upper downstream portion of the muffle 14 as sensed by the upper downstream thermocouple 56.

The controller 70 then performs the second preprogrammed PID function at step 184 using the 1110 degrees F. current set point temperature and the 1100 degrees F. signal (process variable) indicative of the temperature in the upper downstream portion of the muffle 14 to generate a second control signal C₂. Because the sensed temperature is less than the current set point temperature, the second control signal C₂ output at step 184 causes the SCR 82 to use a longer duty cycle, resulting in more power output by the upper downstream heating element 60. At step 186 the control cycle 158 ends until the next wake-up call.

Accordingly, the energy injected into the muffle 14 by the upper downstream heating element 60 is increased such that when the newly loaded work pieces on the conveyer belt 52 approach the downstream portion of the zone 54, the downstream portion of the zone 54 will have been “pre-heated” and will not experience the same temperature excursion below the normal set point temperature as was experienced in the upstream portion of the zone 54. Of course, depending upon various design choices, the actual temperature in the zone 54 may not have increased, but the amount of heat being injected into the muffle 14 will have increased.

The above process is repeated with the control signals being modified in accordance with sensed changes in temperature until the condition set in step 176 is no longer met. Thus, when the sensed temperature at step 164 reaches 1096 degrees F., then at step 176 the sensed temperature will no longer be less than the 1096 degrees F. deviation low set point. Accordingly, the current set point temperature for the downstream portion of the zone 54 is selected as the normal set point temperature stored in the memory slot 148. Accordingly, at step 182, the second PID function will be performed by the controller using the present temperature as sensed by the upper downstream thermocouple 56 and the 1100 degree F. current set point. Thus, the set point temperature for the downstream portion of the zone 54 is lowered once the temperature in the upstream portion has been brought back into the normal operating band.

During the time that the control cycle 158 is generating control signals to control the power passing through the SCRs 80 and 82, the control circuit 68 is also performing over-temperature checks using the upper over-temperature thermocouple 64 and control instrument 96 which operate independently of the controller 70. This independent circuit is provided to protect the zone 54 from an extreme over-temperature condition caused, for example, by a faulty thermocouple or some other device. In the event the temperature in the upper area of the muffle 14 is too high, the high temperature is sensed by the over-temperature thermocouple 64 and a signal indicative of the over-temperature condition is sent to the control instrument 96. In response, the control instrument 96 opens the contacts 88 and 90 in the power supply line to the SCRs 80 and 82, respectively. Accordingly, the energy output of the upper upstream heating element 44 and the upper downstream heating element 60 goes to zero regardless of the first and second control signals generated by the controller 70. The lower over-temperature thermocouple 66 and control instrument 98 work similarly.

Those of skill in the relevant art will appreciate that the present invention may be adapted for use in a number of ways. By way of example, but not of limitation, it may be desired to program the controller such that the downstream temperature current set point is not modified until after the sensed upstream temperature has more closely approached, or even reached, the normal temperature set point for the upstream portion of the zone. One such embodiment which further allows for the normal upstream and downstream temperature set points to be individually controlled is discussed with reference to FIGS. 10, 11 and 12. In this alternative embodiment, the various parameters input by the operator are stored in an external memory for use by the controller 70. As shown in FIG. 10, memory 190 includes memory slots M₁-M₈, identified by reference numbers 191, 192, 193, 194, 195, 196, 197 and 198, respectively. The memory slot 191 is used to store the set point used by a first PID function in the controller 70 in controlling an upstream heater bank. The memory slot 192 is used to store an override output control percentage power value, if any, to be used in controlling the power supplied to the upstream heater bank. The memory slot 193 is used to store an override output control percentage power value, if any, to be used in controlling the power supplied to a downstream heater bank.

The memory slot 194 is used to store the normal set point used by a second PID function in the controller 70 in controlling a downstream heater bank. The memory slot 195 is used to store a value that is added to the set point value in the memory slot 191 in determining if an upstream temperature in the brazing furnace 12 is too hot (the deviation high set point). The memory slot 196 is used to store a set point used by the second PID in controlling the downstream heater bank if the upstream temperature in the zone 54 exceeds the deviation high set point. Accordingly, the value stored in the memory slot 196 will typically be less than the value stored in the memory slot 191.

The memory slot 197 is used to store a value that is subtracted from the set point value in the memory slot 191 in determining if an upstream temperature in the zone 54 is too cold (the deviation low set point). The memory slot 198 is used to store a set point used by the second PID in controlling a downstream heater bank if the upstream temperature in the zone 54 is less than the deviation low set point. Accordingly, the value stored in the memory slot 198 will typically be greater than the value stored in the memory slot 191.

Operation of an alternatively programmed control circuit 68 is described with reference to FIGS. 10 and 11. FIG. 11 shows a control cycle 200 that may be used to control the power generated by an upstream and downstream heater bank in accordance with principles of the present invention. At step 202, the control cycle 200 is initialized and values set by an operator are stored in the memory 190. A current set point pointer is also set to point at memory slot 194. The control cycle 200 then begins at step 204 in response to a wake-up call to the controller 70. At step 206 the value of the signal generated by input/output module 72 is read. The signal generated by input/output module 72 is indicative of the temperature in the upper upstream portion of the zone 54 as sensed by the upper upstream thermocouple 42.

At step 208, the value stored in the memory slot 191 is read. The value stored in the memory slot 191 is the set point temperature established by the operator for the upper upstream portion of the zone 54. The controller 70 at step 210 then checks to see if a valid override output value has been stored in memory slot 192. If there is no valid value stored in the memory slot 192, then at step 212 the controller 70 performs a first preprogrammed PID function using the set point temperature from memory slot 191 and the signal (process variable) indicative of the temperature in the upper upstream portion of the zone 54 from the input/output module 72 to generate a first control signal C₁.

The first preprogrammed PID function in this embodiment also operates to bring the sensed temperature to the set point temperature, and to then maintain the sensed temperature at the set point temperature in a manner understood by those of ordinary skill in the relevant art. The first control signal C₁ corresponds to a percentage output power that should be supplied to the upper upstream heating element 44 in order to reach or maintain the set point temperature stored in the memory slot 191. The first control signal C₁ accomplishes this by controlling the duty cycle of the SCR 80 when the first control signal C₁ is output to the SCR 80 at step 214. The duty cycle corresponds to a percentage output power that is subsequently supplied to the muffle 14 from the upper upstream heating element 44.

Alternatively, returning to step 210, if a valid value is stored in the memory slot 192, then at step 216 that value is read from the memory slot 192 and is set as the first control signal C₁ which is output at step 214 to the SCR 80.

The controller 70 at step 218 then checks to see if a valid override output value has been stored in memory slot 193. If there is no valid value stored in the memory slot 193, then at step 220 the controller 70 determines if the temperature sensed by the upper upstream thermocouple 42 is equal to the set point temperature stored in the memory slot 191 for the upper upstream area of the zone 54. If the two values are equal, then at step 222 the current set point pointer is set to memory slot 194 wherein the normal downstream set point temperature is stored. If the two values are not equal, then at step 224 the controller 70 determines if the temperature sensed by the upper upstream thermocouple 42 is greater than the deviation high set point temperature defined as the temperature stored in the memory slot 191 for the upper upstream area of the zone 54 plus the high deviation value stored in the memory slot 195. If the temperature sensed by the upper upstream thermocouple 42 is greater than the deviation high set point temperature, then at step 226 the current set point pointer is set to the memory slot 96 wherein the current set point temperature for a deviation high condition is stored.

If the temperature sensed by the upper upstream thermocouple 42 is not greater than the deviation high set point temperature, then at step 228 the controller 70 determines if the temperature sensed by the upper upstream thermocouple 42 is less than the deviation low set point temperature defined by the set point temperature stored in the memory slot 191 for the upper upstream area of the zone 54 minus the low deviation value stored in the memory slot 197. If the temperature sensed by the upper upstream thermocouple 42 is less than the deviation low set point temperature, then at step 230 the current set point pointer is set to the memory slot 198 wherein the current set point temperature for a deviation low condition is stored.

At step 232, the controller 70 reads the value in the memory slot to which the current set point pointer has been set as the current set point temperature for a second preprogrammed PID function. The current set point temperature will thus be one of the normal set point temperature in the memory slot 194, the current set point temperature for a deviation low condition in the memory slot 196 or the current set point temperature for a deviation high condition in the memory slot 198. At step 234 the value of the signal generated by input/output module 74 is read. The signal generated by input/output module 74 is indicative of the temperature in the upper downstream portion of the zone 54 as sensed by the upper downstream thermocouple 56.

The controller 70 then performs the second preprogrammed PID function at step 236 using the current set point temperature for the sensed condition and the signal (process variable) indicative of the temperature in the upper downstream portion of the zone 54 to generate a second control signal C₂. Alternatively, if at step 218 a valid value is stored in the memory slot 193, then at step 238 that value is read from the memory slot 193 and is set as the second control signal C₂. At step 240, the second control signal C₂ is output to the SCR 82. The second control signal C₂ and the SCR 82 operate in a manner similar to that described above with respect to the first control signal C₁ and the SCR 80 so as to control the power output of the upper downstream heating element 60.

At step 242 the control cycle 200 ends until the next wake-up call. Upon receiving a wakeup call, the control cycle 200 begins again at step 204.

The simplified operation of the alternatively programmed control circuit 68 is described below as a response to a hypothetical high sensed temperature in the upper upstream portion of the brazing furnace which occurs as a result of the following hypothetical scenario. The brazing furnace 12 has been operating and the memory slots 148-162 have been previously populated with the values shown in FIG. 12. Both the upstream and downstream portions of the zone 54 are initially at a steady state temperature of 1100 degrees F. with a continuous flow of work pieces through the zone 54. A high temperature excursion is then created by discontinuing placement of work pieces on the conveyer belt 52. Thus, the “cooling” effect of the work pieces is no longer introduced into the brazing furnace 12. Accordingly, as the empty portion of the conveyor belt 52 passes into the upstream portion of the zone 54, the temperature in the upstream portion of the zone 54 increases to 1111 degrees F. after the completion of a control cycle 200.

When the next wake-up call is issued, the controller 70 begins the control cycle 200 at step 204. At step 206 the temperature in the upper upstream portion of the zone 54 is read as 1111 degrees F. At step 208 the set point temperature established by the operator in memory slot 191 is read as 1100 degrees F. The controller 70 at step 210 determines that a valid override output value has not been stored in memory slot 192. Therefore, at step 212 the controller 70 performs a first preprogrammed PID function using the set point temperature of 1100 degrees F. from memory slot 191 and the signal (process variable) indicative of the sensed 1111 degrees F. temperature in the upper upstream portion of the zone 54 to generate a first control signal C₁. Because the sensed temperature is greater than the set point temperature, the first control signal C₁ is generated so as to shorten the duty cycle of the SCR 80, resulting in a lower power output from the upper upstream heating element 44. This will lead to a lowering of the temperature in the upper upstream portion of the zone 54 as a result of normal heat loss from the brazing furnace 12.

The controller 70 at step 218 then checks to see if a valid override output value has been stored in the memory slot 193. Because there is no valid value stored in the memory slot 193, then at step 220 the controller 70 determines that the 1111 degrees F. temperature sensed by the upper upstream thermocouple 42 is not equal to the 1100 degrees F. set point temperature stored in the memory slot 191 for the upper upstream area of the zone 54. Accordingly, the controller 70 proceeds to step 224 and determines that the 1111 degrees F. temperature sensed by the upper upstream thermocouple 42 is greater than the 1110 degrees F. deviation high set point temperature defined by the 1100 degrees F. set point temperature stored in the memory slot 191 for the upper upstream area of the zone 54 plus the 10 degrees F. high deviation value stored in the memory slot 195. Therefore, at step 226 the current set point pointer is set to the memory slot 196 wherein the 1090 degrees F. current set point for a deviation high condition is stored.

At step 232, the controller 70 reads the 1090 degrees F. set point in the memory slot 196 as the current set point temperature for a second preprogrammed PID function. At step 234 the value of the signal generated by input/output module 74 is read. The signal generated by input/output module 74 is indicative of an 1100 degrees F. temperature in the upper downstream portion of the zone 54 as sensed by the upper downstream thermocouple 56.

The controller 70 then performs the second preprogrammed PID function at step 236 using the 1090 degrees F. current set point for a high deviation condition and the 1100 degrees F. signal (process variable) indicative of the temperature in the upper downstream portion of the zone 54 to generate a second control signal C₂. Because the sensed temperature is greater than the current set point temperature, the second control signal C₂ causes the SCR 82 to use a shorter duty cycle, resulting in less power output by the upper downstream heating element 60. At step 242 the control cycle 200 ends until the next wake-up call.

Accordingly, the energy injected into the zone 54 by the upper downstream heating element 60 is reduced such that when the empty portion of the conveyer belt 52 approaches the downstream portion of the zone, the downstream portion of the zone 54 will not experience the same temperature excursion above the normal set point (the set point temperature in memory slot 194) as was experienced in the upstream portion of the zone 54.

The above process is repeated with the control signals being modified in accordance with sensed changes in temperature and the current set point pointer used in obtaining the set point temperature for the second PID function will continue to point at the memory slot 196 until one of the conditions set in step 220 and 228 are met or until a manual override is detected. Thus, when the sensed temperature at step 206 reaches 1100 degrees F., then at step 220 the sensed temperature will equal the set point temperature for the upstream upper portion of the zone 54. Accordingly, the controller will proceed to step 222 and set the current set point pointer to the memory slot 194 which has the 1100 degrees F. set point value that is stored for use in normal conditions. Therefore, the set point temperature for the downstream portion of the zone will return to the normal set point temperature once the temperature in the upstream portion has been normalized.

The embodiment of FIG. 11 thus introduces a hysteresis effect into the control of the downstream heater banks which eliminates undesired flutter in the second control signal when the sensed temperature at step 206 is equal to the set point temperature stored in the memory slot 191 for the upper upstream area of the zone 54. The amount of hysteresis, if any, is a design choice. By way of example, but not of limitation, additional temperatures may be defined in the embodiment of FIG. 11 wherein the current set point pointer is directed to the memory slot 194. Accordingly, the controller 70 may be programmed to revert to use of a normal temperature set point as the current temperature set point for use in the second PID function when the sensed temperature in the upstream portion of the zone 54 is at a temperature deviation from the temperature set point between the temperature set point used in the first PID function and the high or low deviation condition temperature. Thus, an optimum hysteresis may be selected which need not be symmetrical with respect to the high or low deviation condition temperature. These modifications and others are within the scope of the present invention.

Continuing with the above simplified example, after re-establishing steady state temperature within the muffle at 1100 degrees F., a cooling excursion may be initiated by subsequent loading of the conveyor belt. Accordingly, as the work pieces begin to enter the upstream portion of the zone 54, the temperature in the upstream portion of the zone 54 begins to decrease. For purposes of this hypothetical, the temperature decreases to 1094 degrees F. after a control cycle 200 has ended.

Subsequently, a wake-up call is issued and the controller 70 begins the control cycle 200 at step 204. At step 206 the temperature in the upper upstream portion of the zone 54 is read as 1094 degrees F. At step 208 the set point temperature established by the operator in the memory slot 191 is read as 1100 degrees F. The controller 70 at step 210 determines that a valid override output value has not been stored in the memory slot 192. Therefore, at step 212 the controller 70 performs a first preprogrammed PID function using the set point temperature of 1100 degrees F. from the memory slot 191 and the signal (process variable) indicative of the 1094 degrees F. temperature in the upper upstream portion of the zone 54 to generate a first control signal C₁. Because the sensed temperature is less than the set point temperature, the first control signal C₁ is generated so as to increase the duty cycle of the SCR 80, resulting in a higher power output from the upper upstream heating element 44. This will lead to an increase of the temperature in the upper upstream portion of the zone 54.

The controller 70 at step 218 then checks to see if a valid override output value has been stored in the memory slot 193. Because there is no valid value stored in the memory slot 193, then at step 220 the controller 70 determines that the 1096 degrees F. temperature sensed by the upper upstream thermocouple 42 is not equal to the 1100 degrees F. set point temperature stored in the memory slot 191 for the upper upstream area of the zone 54. Accordingly, the controller 70 proceeds to step 224 and determines that the 1096 degrees F. temperature sensed by the upper upstream thermocouple 42 is not greater than the 1100 degrees F. set point temperature stored in the memory slot 191 for the upper upstream area of the zone 54 plus the 10 degrees F. high deviation value stored in the memory slot 195. The controller 70 then proceeds to step 228 and determines that the 1096 degrees F. temperature sensed by the upper upstream thermocouple 42 is less than the 1100 degrees F. set point temperature stored in the memory slot 191 for the upper upstream area of the zone 54 minus the 5 degrees F. high deviation value stored in the memory slot 197. Therefore, at step 230 the current set point pointer is set to the memory slot 198 wherein the 1110 degrees F. current set point for a sensed deviation low condition is stored.

At step 232, the controller 70 reads the 1110 degrees F. value in the memory slot 198 as the current set point temperature for the second preprogrammed PID function. At step 234 the value of the signal generated by input/output module 74 is read. The signal generated by input/output module 74 is indicative of an 1100 degrees F. temperature in the upper downstream portion of the zone 54 as sensed by the upper downstream thermocouple 56.

The controller 70 then performs the second preprogrammed PID function at step 236 using the 1110 degrees F. current set point temperature and the 1100 degrees F. signal (process variable) indicative of the temperature in the upper downstream portion of the zone 54 to generate a second control signal C₂. Because the sensed temperature is less than the current set point temperature, the second control signal C₂ causes the SCR 82 to use a longer duty cycle, resulting in more power output by the upper downstream heating element 60. At step 242 the control cycle 200 ends until the next wake-up call.

Accordingly, the energy injected into the zone 54 by the upper downstream heating element 60 is increased such that when the newly loaded work pieces on the conveyer belt 52 approach the downstream portion of the zone 54, the downstream portion of the zone 54 will have been “pre-heated” and will not experience the same temperature excursion below the normal set point temperature as was experienced in the upstream portion of the zone 54.

The above process is repeated with the control signals being modified in accordance with sensed changes in temperature and the current set point pointer used in obtaining the set point temperature for the second PID function will continue to point at the memory slot 198 until one of the conditions set in step 220 and 224 are met or until a manual override is detected. Thus, when the sensed temperature at step 206 reaches 1100 degrees F., then at step 220 the sensed temperature will equal the set point temperature for the upstream upper portion of the zone 54. Accordingly, the controller will proceed to step 222 and set the current set point pointer to the memory slot 194 which has the 1100 degrees F. set point temperature that is stored for use in normal conditions. Therefore, the set point temperature for the downstream portion of the zone 54 is lowered once the temperature in the upstream portion has been normalized.

While the invention has been illustrated and described in detail in the drawings and the foregoing description, the same should be considered as illustrative and not restrictive in character. It is understood that only the preferred embodiments have been presented and that all changes, modifications and further applications that come within the spirit of the invention are desired to be protected. 

1. A continuous control atmosphere brazing furnace comprising: a muffle having a first area and a second area; a carrier operable to convey an item from the first area to the second area; a first heating element operable to provide heat energy to the first area as a function of a first control signal; a second heating element operable to provide heat energy to the second area as a function of a second control signal; a first thermocouple operable to sense the temperature within the first area; a second thermocouple operable to sense the temperature within the second area; and a controller operably connected to the first heating element, the second heating element, the first thermocouple and the second thermocouple and programmed to perform a first comparison of the sensed temperature from the first thermocouple with a first set point temperature, generate the first control signal based upon the first comparison, determine a current set point based upon the first comparison, perform a second comparison of the sensed temperature from the second thermocouple with the current set point temperature, and generate the second control signal based upon the second comparison.
 2. The continuous control atmosphere brazing furnace of claim 1, wherein the controller is further programmed to: determine the current set point to be a low deviation current set point when the sensed temperature from the first thermocouple is less than the first set point by a first predetermined amount.
 3. The continuous control atmosphere brazing furnace of claim 2, wherein the controller is further programmed to: determine the current set point to be a high deviation current set point when the sensed temperature from the first thermocouple is greater than the first set point by a second predetermined amount; and determine the current set point to be a normal current set point when the sensed temperature from the first thermocouple is not determined to be less than the first set point by a first predetermined amount or greater than the first set point by a second predetermined amount.
 4. The continuous control atmosphere brazing furnace of claim 2, wherein the controller is programmed to: perform the first comparison using a first proportional-integral-derivative function; and perform the second comparison using a second proportional-integral-derivative function.
 5. The continuous control atmosphere brazing furnace of claim 4, further comprising: a first silicon controlled rectifier located between the controller and the first heating element and operable to receive the first control signal and to provide power to the first heating element as a function of the first control signal; and a second silicon controlled rectifier located between the controller and the second heating element and operable to receive the second control signal and to provide power to the second heating element as a function of the first control signal.
 6. The continuous control atmosphere brazing furnace of claim 5, further comprising: an over-temperature probe located within an upper portion of the furnace and operable to generate a signal corresponding to a sensed over temperature condition within the muffle; an over-temperature instrument operable to receive the signal from the first over-temperature probe and to interrupt power to the first and the second silicon controlled rectifiers when an over-temperature condition in the muffle is sensed.
 7. The continuous control atmosphere brazing furnace of claim 1, wherein: the muffle comprises a third and a fourth area; the carrier belt is further operable to convey an item from the third area to the fourth area; the furnace further comprises a third heating element operable to provide heat energy to the third area as a function of a third control signal, a fourth heating element operable to provide heat energy to the fourth area as a function of a fourth control signal, a third thermocouple operable to sense the temperature within the third area, a fourth thermocouple operable to sense the temperature within the fourth area, and the controller is further programmed to perform a third comparison of the sensed temperature from the third thermocouple with a third set point temperature, generate the third control signal based upon the third comparison, determine a second current set point based upon the third comparison, perform a fourth comparison of the sensed temperature from the fourth thermocouple with the second current set point, and generate the fourth control signal based upon the fourth comparison.
 8. The continuous control atmosphere brazing furnace of claim 7, wherein: the muffle further comprises an upper portion, a lower portion, an inlet and an outlet downstream of the inlet; the first area is about one third of the distance from the inlet to the outlet in the upper portion of the muffle; the second area is about two thirds of the distance from the inlet to the outlet in the upper portion of the muffle; the third area is about one third of the distance from the inlet to the outlet in the lower portion of the muffle; and the fourth area is about two thirds of the distance from the inlet to the outlet in the lower portion of the muffle.
 9. The continuous control atmosphere brazing furnace of claim 7, further comprising: a first silicon controlled rectifier located between the controller and the first heating element and operable to receive the first control signal and to provide power to the first heating element as a function of the first control signal; a second silicon controlled rectifier located between the controller and the second heating element and operable to receive the second control signal and to provide power to the second heating element as a function of the first control signal; a third silicon controlled rectifier located between the controller and the third heating element and operable to receive the first control signal and to provide power to the third heating element as a function of the third control signal; and a fourth silicon controlled rectifier located between the controller and the fourth heating element and operable to receive the fourth control signal and to provide power to the fourth heating element as a function of the fourth control signal.
 10. The continuous control atmosphere brazing furnace of claim 7, wherein: the muffle comprises a first zone and a second zone, the second zone located downstream of the first zone; the first and second areas are located within the first zone; and the third and fourth areas are located within the second zone.
 11. A furnace system comprising: a furnace with a first area and a second area; a first thermocouple operable to sense the temperature within the first area and to generate a signal indicative of the sensed temperature; a second thermocouple operable to sense the temperature within the second area and to generate a signal indicative of the sensed temperature; a first heating element with a variable heat output located proximate the first area and operable to affect the temperature within the first area; a second heating element with a variable heat output located proximate the second area and operable to affect the temperature within the second area; a controller, operable to receive the signal from the first thermocouple and the signal from the second thermocouple and programmed to compare the signal from the first thermocouple to a first set point, and modify a second set point based on the comparison of the signal from the first thermocouple to the first set point, wherein the first set point is used in determining the amount of heat to be provided to the first area and the second set point is used in determining the amount of heat to be provided to the second area.
 12. The furnace system of claim 11, further comprising: an over-temperature probe located within an upper portion of the furnace and operable to sense an over-temperature condition in the upper portion of the furnace; an over-temperature instrument operably connected to the first over-temperature probe and operable to disable the first heating element and the second heating element when an over-temperature condition is sensed in the upper portion of the furnace.
 13. The furnace system of claim 11, wherein the controller is programmed to: compare the signal from the first thermocouple to the first set point using a first proportional-integral-derivative function; generate a first control signal based upon the comparison by the first proportional-integral-derivative function; compare the signal from the second thermocouple to the modified second set point using a second proportional-integral-derivative function; and generate a second control signal based upon the comparison by the second proportional-integral-derivative function.
 14. A method of controlling temperature within a furnace comprising: defining a first temperature set point for a first area in a furnace; obtaining a first signal indicative of the temperature in the first area of the furnace; comparing the first signal to the first temperature set point; controlling the temperature in the first area based upon the comparison of the first signal to the first temperature set point; defining a second temperature set point for a second area in the furnace; defining a third temperature set point for the second area in the furnace selecting one of the second temperature set point or the third temperature set point based upon the comparison of the first signal to the first temperature set point; obtaining a second signal indicative of the temperature in the second area of the furnace; comparing the second signal to the selected temperature set point; and controlling the temperature in the second area based upon the comparison of the second signal to the selected temperature set point.
 15. The method of claim 14, wherein: the step of comparing the first signal to the first temperature set point comprises determining when the first signal indicates that the temperature in the first area is lower than the first temperature set point by an amount greater than about a first deviation value; the step of defining a third temperature set point comprises defining the third temperature set point to be value greater than the second temperature set point; and the step of selecting comprises selecting the third temperature set point when it is determined that the first signal indicates that the temperature in the first area is lower than the first temperature set point by an amount greater than about the first deviation value and selecting the second temperature set point when it is determined that the first signal indicates that the temperature in the first area is not lower than the first temperature set point by an amount greater than about the first deviation value.
 16. The method of claim 14, wherein: the step of comparing the first signal to the first temperature set point comprises determining when the first signal indicates that the temperature in the first area is higher than the first temperature set point by an amount greater than about a second deviation value; the step of defining a third temperature set point comprises defining the third temperature set point to be a value greater than the second temperature set point; and the step of selecting comprises selecting the second temperature set point when it is determined that the first signal indicates that the temperature in the first area is higher than the first temperature set point by an amount greater than about the second deviation value and selecting the third temperature set point when it is determined that the first signal indicates that the temperature in the first area is not higher than the first temperature set point by an amount greater than about the first deviation value.
 17. The method of claim 14, further comprising the step of defining a fourth temperature set point for the second area in the furnace to be a value less than the second temperature set point, wherein: the step of comparing the first signal to the first temperature set point comprises 1) determining when the first signal indicates that the temperature in the first area is higher than the first temperature set point by an amount greater than about a third deviation value and 2) determining when the first signal indicates that the temperature in the first area is lower than the first temperature set point by an amount greater than about a fourth deviation value; the step of defining a third temperature set point comprises defining the third temperature set point at a temperature greater than the second temperature set point; and the step of selecting comprises 1) selecting the fourth temperature set point when it is determined that the first signal indicates that the temperature in the first area is higher than the first temperature set point by an amount greater than about the third deviation value, 2) selecting the third temperature set point when it is determined that the first signal indicates that the temperature in the first area is lower than the first temperature set point by an amount greater than about the fourth deviation value, and 3) selecting the second temperature set point when it is determined that the first signal indicates that the temperature in the first area is (a) greater than the remainder of the first temperature set point minus the fourth deviation value, and (b) less than the sum of the first temperature set point and the third deviation value.
 18. The method of claim 17, further comprising after the step of selecting the third temperature set point: maintaining the third temperature set point as the selected temperature set point until it is determined that the first signal indicates that the temperature in the first area is greater than the remainder of the first temperature set point minus the fourth deviation value by a fifth deviation value.
 19. The method of claim 14, wherein: the step of obtaining a first signal indicative of the temperature in the first area of the furnace comprises obtaining the first signal when an item on a conveyer belt is within the first area; and the step of controlling the temperature in the second area based upon the comparison of the second signal to the selected temperature set point comprises controlling the temperature in the second area to be at about the selected temperature set point at about the time that the item present in the first area at the time the first signal indicative of the temperature in the first area of the furnace was obtained is conveyed into the second area.
 20. The method of claim 14, further comprising defining a fifth temperature set point for a third area in a furnace; obtaining a third signal indicative of the temperature in the third area of the furnace; comparing the third signal to the fifth temperature set point; controlling the temperature in the third area based upon the comparison of the third signal to the fifth temperature set point; defining a sixth temperature set point for a fourth area in the furnace; defining a seventh temperature set point for the fourth area in the furnace selecting one of the sixth temperature set point or the seventh temperature set point as a second selected temperature set point based upon the comparison of the third signal to the fifth temperature set point; obtaining a fourth signal indicative of the temperature in the fourth area of the furnace; comparing the fourth signal to the second selected temperature set point; and controlling the temperature in the fourth area based upon the comparison of the fourth signal to the second selected temperature set point.
 21. A method of controlling temperature within a conveyer type furnace comprising: defining a first temperature set point for a first area in a furnace; obtaining a first signal indicative of the temperature in the first area of the furnace; comparing the first signal to the first temperature set point; controlling the temperature in the first area based upon the comparison of the first signal to the first temperature set point; defining a second temperature set point for a second area in the furnace; determining if the first signal indicates a temperature greater than the first temperature set point by a first deviation value; decreasing the second temperature set point if the first signal indicates a temperature greater than the first temperature set point by the first deviation value; obtaining a second signal indicative of the temperature in the second area of the furnace; comparing the second signal to the decreased second temperature set point; and controlling the temperature in the second area based upon the comparison of the second signal to the decreased second temperature set point.
 22. The method of claim 21, further comprising: determining if the first signal indicates a temperature less than the first temperature set point by a second deviation value; increasing the second temperature set point if the first signal indicates a temperature less than the first temperature set point by the second deviation value; comparing the second signal to the increased second temperature set point; and controlling the temperature in the second area based upon the comparison of the second signal to the increased second temperature set point. 